Epoxy adhesives having pressure-sensitive properties may be prepared by addition of matrix polymers. Adhesive bond strength often increases with increasing amount of epoxy. Since most epoxies are used in liquid adhesives they have a relatively low viscosity and high epoxy contents thus do not afford pressure-sensitive tapes. High-viscosity/solid epoxies are required therefor.
The viscosity of epichlorohydrin-based epoxy resins is typically adjusted via the ratio of epichlorohydrin and a resulting molecular weight increase at the expense of epoxy concentration [“Epoxy Adhesive Formulations” 2006, E. M. Petrie, p. 30ff]. A number of examples such as epoxy (cresol) novolacs are produced via a different synthetic route and carry the epoxy groups as a side group so that an increase in molecular weight does not lead to a reduction in epoxy concentration.
However apart from the epoxy novolacs referred to hereinabove and the oligomers produced with epichlorohydrin, there are no processes for obtaining epoxies, in particular also epoxies other than those mentioned, in a high-viscosity state.
More specialized epoxy monomers, for example epoxycyclohexyl derivatives, which are not produced via the epichlorohydrin route are accordingly often very liquid (e.g. Uvacure 1500). Since many epoxies are used for crosslinking reaction (they contain more than one epoxy group per molecule) they cannot simply be polymerized to increase viscosity since crosslinked and insoluble deposits are immediately formed. These epoxycyclohexyl monomers are preferably employed in cationically curable adhesives, in particular for UV-induced curing via photolabile acid generators.
There is therefore a need for a method of increasing the viscosity of epoxies not obtained via the epichlorohydrin route.
AU 758128 B2 describes such storage-stable cationically curing multifunctional epoxy mixtures. To achieve storage stability of the multifunctional epoxy resins it is necessary according to this document to avoid undesired premature polymerization. This is achieved by adding 0.0005 to 10 wt % of an organic and/or inorganic alkaline earth metal or alkali metal compound during the polymerization of multifunctional epoxy monomers. Polymerization before desired crosslinking is thus to be avoided in any event. In addition to the inventive stabilization of these adhesives it follows from this document that for cationically curable liquid adhesives epoxycyclohexyl derivatives are particularly preferred so that it would be desirable to be able to produce high-viscosity epoxies from this monomer as well.
SG 160949 B proposes reacting diepoxies with diisocyanates. This mixture affords cyclic trimers (isocyanurates) and oxazolidinones having an Mw<3000 g/mol and a low polydispersity which can then be cured like epoxies. It is necessary to use at least difunctional substances since otherwise no reactive groups would be available for the subsequent curing. However, the compounds formed have low viscosities.
The present invention accordingly has for its object the provision of a process which makes it possible to produce specifically viscous adhesive resins from originally liquid epoxies. The product shall be soluble in solvents and meltable by the temperature being raised.